concrete slab on grade point loading: Enercalc vs. Ringo & Anderson

[mrsutton]

I realize slab on grade point loading may be an old topic but has anyone noticed an order of magnitude discrepancy between Enercalc slab on grade capacity compared to the capacities given with the Ringo & Anderson formula?

Any opinion as to which is "more correct": the Enercalc formula, if I remember their help commentary, is based on FEA results while the Ringo & Anderson formula is empirical (and probably very very conservative). Enercalc results are on the order of 10x the capacity given by Ringo & Anderson.

 

[Hokie93]

The article "Load-Carrying Capacity: Concrete Slabs-on-Grade Subject to Concentrated Loads" by Azzi and Laird from the April 2008 issue of STRUCTURE magazine provides solutions that might help you determine which of your results is correct. The article is available from the STRUCTURE magazine website at:

http://www.structuremag.org/wp-content/uploads/2014/08/C-StructuralDesign-Azzi-Apr081.pdf.

 

[manstrom]

Enercalc is not always right. It is simple enough that you can check their calcs by hand. I have seen several modules giving incorrect (but conservative) information.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[hawkaz]

Enercalc uses the Shentu method. This gives much higher allowable loading

 

[Once20036]

As Hokie93 said, the enercalc method is based on the structure magazine equations.

I've read through the article and my primary complaint is that they never indicate what they consider to be a failure of the slab on grade. As best I can tell, they do not consider settlement or cracking to be a failure, just the inability of the slab to continue carrying load.
My second complaint is that I cannot track down several of their cited references, most notable reference 9 from the SEAOSC. I've had several conversations with SEAOSC and nobody has been able to find a copy of this.

Personally, I tend to look at where the slab is and what I would consider to be an acceptable risk.
I look at the Ringo results, I look at the Structure magazine results, and I look at the British TR-34 results, and make a judgement call.

 

[mrsutton]

I found at least a summary of the primary academic paper referenced in both Structure magazine and Enercalc help. The best I could tell was the formula was developed based on a best-fit regression of a bunch of FEA model slabs on grade loaded to failure--not real slabs. I'm sure Ringo being older is just super conservative.

It's unnerving to tell clients the need to cut in footings for posts (say for a new mezzanine in a warehouse) based on Ringo numbers, but then have Enercalc results which show the slab DCR at 15%.